Shock absorbers are widely applied to damp motion between two parts, for instance, in cars, trains and other vehicles to damp a motion of the body with respect to the wheels. A main non-return valve is generally designed such that it only opens at a predetermined pressure difference across the valve and such as to have a predefined stiffness to provide damping of fluid flow between working chambers of the shock absorber. This causes a damping behavior of movements of the vehicle that can be considered as being uncomfortable for persons in the vehicle since it sets in very abrupt. Especially when a shock absorber is designed to provide a stiff damping characteristic it is conceived as not being very comfortable.
The predetermined preload of a non-return valve on the valve seat is required for noise reduction and determines a closing point in a transient from bump to rebound and from rebound to bump. This influences the damper performance and ride properties, and requires an improved solution to provide a better performance. Shock absorbers requiring a damping characteristic for both bump and rebound (inward and outward damping) conceptually generate a bump force that is generally ten times stiffer than in a standard twin tube configuration (proportional to (rod piston surface divided rod surface)2). A return construction with slow closing feature but not requiring a preload for return movement is key to ride improvement and noise reduction.
EP 2 108 858 A2 discloses shock absorbers having a four or even five-stage damping characteristic for both inward and outward movements of the piston with respect to the cylinder of the shock absorber. The FIG. 3 embodiment of the publication presents a five-stage damping characteristic. The piston separates the cylinder into two working chambers and has an inward valve and an outward valve that each provide for a direct connection between both working chambers in the fifth (and last) damping stage of the shock absorber to allow for large fluid volume flows in between working chambers. The piston further comprises an auxiliary housing defining a cavity divided into two auxiliary chambers by a separating element that can move within the auxiliary housing. Each auxiliary chamber is by a respective opening in fluid communication with an associated working chamber of the shock absorber, and a bypass channel is provided on the internal surface of the auxiliary housing to provide a fluid connection between both auxiliary chambers when the opening of one of the auxiliary chambers to its associated working chamber is closed by the separating element. Each auxiliary chamber has a further opening with a valve to its associated working chamber, and openings with valves are provided on the separating element to allow fluid flow through the separating element between both auxiliary chambers. The arrangement of the auxiliary housing with its various elements allows for four damping stages, while the fifth damping stage comes in parallel to the flow through the auxiliary housing at large fluid volume flows between both working chamber. The shock absorber provides for a complex multi-stage damping behavior with noticeable transitions between damping stages. The construction of the shock absorber is also very complex and requires careful tuning of the various components, especially in relation to one another to achieve the required damping behavior. The design requires a rather long length of the piston and therefore an increased length of the shock absorber or a decreased stroke length at a same shock absorber length.
US 2015/0152936 A1 discloses a shock absorber having a connection channel between both sides of the piston and thus between compression and rebound chambers. An auxiliary piston valve assembly is arranged in the connection channel and closes the connection channel in dependency of frequency and stroke length. The auxiliary valve assembly provides for a sudden closure to switch between damping modes. The publication indicates that the shock absorber would be frequency-sensitive. However, the construction is such the frequency-sensitivity is quite badly defined. The behavior of the shock absorber will depend both on frequency and stroke length.
DE 103 21 351 A1 also discloses a shock absorber having a connection channel between both sides of the piston. The piston comprises a damping piston and a compensation piston. The compensation piston closes the channel very abruptly at large-amplitude oscillations to switch between damping modes. Large-amplitude damping will set in very abruptly and not provide a comfortable driving experience.